This invention relates to optically active materials and their use as doping agents for liquid crystals for a wide range of applications including solid state cholesteric filters for projection displays, circular polariser, optical filter, etc.
The addition of an optically-active compound to a non-optically-active liquid crystalline phase is one of procedures used for the conversion of non-optically-active into optically-active mesophases. The nematic phase, for example, is converted to the cholesteric one when being doped with a small amount of an optically-active substance. This conversion manifests itself by the occurrence of an intermolecular helix which is characterised by the so-called helical twisting power (HTP) given in Equation 1:                               HTP          =                                                                      "LeftBracketingBar"                                                            ⅆ                                              p                                                  -                          1                                                                                                            ⅆ                      x                                                        "RightBracketingBar"                                                  x                  =                  0                                            ≅                                                p                                      -                    1                                                  x                                      =                                          ∑                i                            ⁢                                                x                  i                                ⁡                                  (                  HTP                  )                                                                    ,                            (        1        )            
Said HTP is in fact a measure for the efficiency of a given dopant and is determined by the Cano method with solutions of the dopant in the host mesophase. Since the optically-active guest and the non-optically-active host compounds are not necessarily compatible at the molecular scale, their binary solution is frequently characterised by undesirable changes of the thermo-tropic sequence of the initial host liquid crystalline material, like for example a depression of the clearing point. Those changes could in turn have negative effects on the phase properties of the host, such as a decrease of the birefringence etc. Therefore, an optically-active dopant is sought so that with very small concentrations of this latter, large values of HTP could be induced.
As such efficient optically-active dopants there are the binaphthol derivatives described in GB-A-2 298 202. However optically-active binaphthol derivatives may undergo partial racemisation when being heated. Besides, their preparation is expensive because it includes asymmetric resolution of binaphthol racemate as a crucial reaction step.
Another type of optically-active dopants are the TADDOLs derivatives (xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetraaryl-1,3-dioxolane-4,5-dimethanols). They have been known for 15 years in asymmetric syntheses (D. Seebach et al., J. Org. Chem. 60, 1788, (1995) and Helv. Chim. Acta 80, 2515, (1997)) and recently investigated as dopants in nematic host mesophases and found to induce large values of HTP (H. G. Kuball et al., Helv. Chim. Acta 80, 2507, (1997) and International Patent Application WO-A-97/34886). This efficiency of TADDOLs compounds was essentially attributed to their conformational stability due to the presence of the dioxolane ring.
Tet. Lett. 36(11), 1995, 1827-1830 (Dube et al.) discloses the compound (2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol as an intermediate in asymmetric synthesis: 
However, no activity is given for the compound.
EP-A-0233602 (Hoechst AG) relates to chiral dopants for liquid crystals. The compounds are all derivatives of butane-1,2,3,4-tetraol of general formula: 
having no further substituents on the butane backbone.
It has been suggested that the large HTP observed with TADDOLs compounds is due to their conformational stability. However we have now discovered that further compounds that exhibit overcrowding of the chiral atoms and are sterically hindered at positions analogous to the xcex1,xcex1xe2x80x2 positions in TADDOLs are equally effective at producing a large HTP.
Thus the invention provides optically active butane-1,2,3,4-tetraol derivatives of formula I: 
in which
A1, A2, A3, A4 each independently represents hydrogen; or an optionally-substituted methyl group; or an optionally-substituted aliphatic group with 2 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom or a group xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94 or xe2x80x94SOSxe2x80x94; or an optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring system, with 1 to 80 C-atoms; and
A5, A6, A7, A8 each independently represents an optionally-substituted aliphatic group with 3 to 80 C-atoms, in which a C-atom, or two or more non-adjacent C-atoms, may be replaced by a heteroatom or a group xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94 or xe2x80x94SOSxe2x80x94; or an optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring system, with 1 to 80 C-atoms; or
one, two or three of A5, A6, A7 and A8 independently represents hydrogen; or an optionally-substituted methyl group; or an optionally-substituted aliphatic group with 2 C-atoms, in which a C-atom may be replaced by a heteroatom; and the remainder of A5, A6, A7, A8 independently represent an optionally-substituted aliphatic group with 3 to 80 C-atoms, in which a C-atom, or two or more non-adjacent C-atoms, may be replaced by a heteroatom, or a group xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94 or xe2x80x94SOSxe2x80x94; or an optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring system, with 1 to 80 C-atoms;
with the provisos that:
when one, two or three of A5, A6, A7 and A8 represents hydrogen, then A5, A6, A7 and A8 may not represent COOH; and
when A5, A6, A7 and A8 are all phenyl, then at least one of A1, A2, A3, A4 does not represent hydrogen.
The term xe2x80x9caliphaticxe2x80x9d includes straight-chain and branched alkyl, as well as saturated and unsaturated groups. Possible substituents include alkyl, aryl (thus giving an araliphatic group) and cycloalkyl, as well as amino, cyano, epoxy, halogen, hydroxy, nitro, oxo etc. Possible heteroatoms which may replace carbon atoms include nitrogen, oxygen and sulphur. In the case of nitrogen further substitution is possible with groups such as alkyl, aryl and cycloalkyl.
In the case of the present invention, owing to the absence of the dioxolane ring, the new optically active butane-1,2,3,4-tetraol derivatives of formula I have obviously more conformational freedom then TADDOLs derivatives. However they were surprisingly found still to induce large values of HTP when being used as dopants in nematic host systems, as will be demonstrated in the forthcoming sections.
The compounds of the invention may be used as doping agents for liquid crystals for a wide range of applications including solid state cholesteric filters for projection displays, circular polarisers, optical filters, etc.
Although one or more of A5, A6, A7 and A8 may represent a small group such as hydrogen or an optionally-substituted methyl group, we prefer there to be two bulkier groups, A5 and A6, attached to the same carbon atom. Thus preferably only A7 and A8 may represent hydrogen. More preferably only one of A5, A6, A7 and A8 may represent hydrogen. The best compounds are those which have four bulky groups. Thus, preferably A5, A6, A7 and A8 each independently represents an optionally-substituted aliphatic group with 3 to 80 C-atoms, in which a C-atom, or two or more non-adjacent C-atoms, may be replaced by a heteroatom or a group xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94 or xe2x80x94SOSxe2x80x94; or an optionally-substituted aromatic or non-aromatic carbocyclic or heterocyclic ring system, with 1 to 80 C-atoms. Carbocyclic or heterocyclic ring system having two or more fused rings have been found to be particularly suitable.
The invention further relates to optically active butane-1,2,3,4-tetraol derivatives of formula I, in which:
A1, A2, A3, A4 each independently represents hydrogen; or an optionally-substituted methyl group; or an optionally-substituted aliphatic group with 2 to 80 C-atoms, in which one or more C-atoms may be replaced by a heteroatom or a group xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94 or xe2x80x94SOSxe2x80x94; or an optionally-substituted aromatic ring system, with 1 to 80 C-atoms; and
A5, A6, A7, A8 each independently represents an optionally-substituted aliphatic group with 3 to 80 C-atoms, in which a C-atom, or two or more non-adjacent C-atoms, may be replaced by a heteroatom or a group xe2x80x94SOxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94 or xe2x80x94SOSxe2x80x94; or an optionally-substituted aromatic ring system, with 1 to 80 C-atoms;
with the proviso that, when A5, A6, A7 and A8 are all phenyl, then at least one of A1, A2, A3, A4 does not represent hydrogen.
Particularly interesting applications are possible if at least one of the A1 to A6 residues includes a polymerisable group.
Preferred embodiments of the invention relates to optically active butane-1,2,3,4-tetraol derivatives of formula I, wherein:
A5 to A8 have each independently one of the meanings of formula II:
xe2x80x94X1xe2x80x94(Sp1)nxe2x80x94X2xe2x80x94(MG)xe2x80x94X3xe2x80x94(Sp2)mxe2x80x94Pxe2x80x83xe2x80x83(II)
A1 to A4 are hydrogen atoms or have each independently one of the meanings of formula IIb or one of the meanings of formula IIc:
xe2x80x94(Sp1)nxe2x80x94X2xe2x80x94(MG)xe2x80x94X3xe2x80x94(Sp2)mxe2x80x94Pxe2x80x83xe2x80x83(IIb)
xe2x80x94(Sp1)nxe2x80x94X2xe2x80x94(MG)xe2x80x94X4xe2x80x83xe2x80x83(IIc)
in which:
X1 to X3 each independently denote xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94N(CH3)xe2x80x94, xe2x80x94CH(OH)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2(CO)xe2x80x94, xe2x80x94SOxe2x80x94, xe2x80x94CH2(SO)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94CH2(SO2)xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94, xe2x80x94SOOxe2x80x94, xe2x80x94OSOxe2x80x94, xe2x80x94SOSxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond, in such a manner that oxygen atoms are not linked directly to one another;
X4 is a halogen;
Sp1 and Sp2 are each independently straight or branched spacer groups having 1 to 20 C-atoms which may be unsubstituted, or mono- or polysubstituted by halogen or CN, it being also possible for one or more non-adjacent CH2 groups to be replaced, in each case independently from one another, by xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94NHxe2x80x94, xe2x80x94N(CH3)xe2x80x94, xe2x80x94CH(OH)xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2(CO)xe2x80x94, xe2x80x94CH2(SO)xe2x80x94, xe2x80x94CH2(SO2)xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94OCOxe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94COxe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94(CF2)xe2x80x94r, xe2x80x94(CD2)sxe2x80x94 or xe2x80x94C(W1)xe2x95x90C(W2)xe2x80x94, r and s ranging between 1 and 15, and W1 and W2 each independently denoting H, Hxe2x80x94(CH2)qxe2x80x94 or Cl with q ranging between 1 and 15;
P is hydrogen or preferably a polymerisable group selected from the formulae CH2xe2x95x90CWxe2x80x94, CH2xe2x95x90CWxe2x80x94COOxe2x80x94, CH2xe2x95x90C(Ph)xe2x80x94COOxe2x80x94, CH2xe2x95x90CHxe2x80x94COOxe2x80x94Phxe2x80x94, CH2xe2x95x90CWxe2x80x94COxe2x80x94NHxe2x80x94, CH2xe2x95x90C(Ph)xe2x80x94CONHxe2x80x94, CH2xe2x95x90C(COORxe2x80x2)xe2x80x94CH2xe2x80x94COOxe2x80x94, CH2xe2x95x90CHxe2x80x94Oxe2x80x94, CH2xe2x95x90CHxe2x80x94OOCxe2x80x94, Phxe2x80x94CHxe2x95x90CHxe2x80x94, CH3xe2x80x94Cxe2x95x90Nxe2x80x94(CH2)m3, HOxe2x80x94, HSxe2x80x94, HOxe2x80x94(CH2)m3xe2x80x94, HSxe2x80x94(CH2)m3xe2x80x94, HO(CH2)m3COOxe2x80x94, HS(CH2)m3COOxe2x80x94, HWNxe2x80x94, HOC(O)xe2x80x94, CH2xe2x95x90CHxe2x80x94Phxe2x80x94(O)m4, 
with W being H, Cl or alkyl with 1-5 C atoms, m3 being 1-9, m4 being 0 or 1, Ph being phenyl, Rxe2x80x2 being alkyl with 1-5 C atoms, and Rxe2x80x3 HAVING the meaning or Rxe2x80x2 or being methoxy, cyano or a halogen;
n and m are each independently 0 or 1; and
MG denotes a mesogenic group comprising 1 to 4 aromatic or non-aromatic carbocyclic or heterocyclic ring systems and optionally up to 3 bridging groups, and preferred are those selected from the meanings of formula III:
xe2x80x83C1"Parenopenst"Z1xe2x88x92C2)al"Parenopenst"Z2xe2x88x92C3)a2"Parenopenst"Z3xe2x88x92C4)a3 xe2x80x83xe2x80x83(III),
in which:
C1 to C4 are in each case independently optionally-substituted non-aromatic, aromatic, carbocyclic or heterocyclic groups;
Z2 to Z3 are independently from each other xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond; and
a1, a2 and a3 are independently integers 0 to 3, such that a1+a2+a3xe2x89xa64.
Especially preferred are those in which C1 to C4 are selected from: 
with:
L being xe2x80x94CH3, xe2x80x94COCH3, xe2x80x94NO2, CN, or halogen,
u1 being 0, 1, 2, 3, or 4,
u2 being 0, 1, 2, or 3, and
u3 being 0, 1, or 2.
We have discovered that, to attain high values of HTP for a given guest-host liquid crystalline system, it is desirable to increase the size and the conformational stability of the substituents around the chiral centre(s) of the optionally active host molecule(s). It is possible to realise such a strategy in the present invention with optically active butane-1,2,3,4-tetraol derivatives of formula I where the two 1 and 4 positions could be derivatised with bulky A5 to A9 substituents starting, for example, from the Grignard analogues of A5 to A8 of appropriate structure, selected from the formula II and tartaric acid, commercially available in both (R,R) and (S,S) enantiomeric forms. The four generated hydroxy groups could be then derivatised, using classical synthetic methods, with A1 to A4 appropriately selected from the formula IIb as organic residues permitting the increase of solubility and/or the increase of compatibility of I with the guest liquid crystalline systems.
More preferred embodiments of the present invention are:
a) for ease of synthesis, optically active butane-1,2,3,4-tetraol derivatives of formula I wherein:
A1 and A2 are identical, and
A3 and A4 are identical, and
A5 and A8 are identical; and
b) optically active butane-1,2,3,4-tetraol derivatives of formula I wherein:
A3 and A4 have one of the meanings of formula IV:
(Sp2)m4xe2x80x94(O)m5xe2x80x94P2xe2x80x83xe2x80x83(IV)
A1 and A2 have one of the meanings of formula VA:
xe2x80x94X5xe2x80x94MGxe2x80x94X3xe2x80x94(Sp2)m4xe2x80x94P3xe2x80x83xe2x80x83(VA)
and
A5 to A8 have one of the meanings of formula VB:
xe2x80x94MGxe2x80x94X3xe2x80x94(Sp2)m4xe2x80x94P3xe2x80x83xe2x80x83(VB)
in which:
Sp2 is alkylene with 0 to 20 C-atoms,
P2 is H, CH2xe2x95x90CW5xe2x80x94 or CH2xe2x95x90CW5xe2x80x94COxe2x80x94,
P3 is H, CH2xe2x95x90CW5xe2x80x94, CH2xe2x95x90CW5xe2x80x94COOxe2x80x94, W5CHxe2x95x90CHxe2x80x94Oxe2x80x94 or CH2xe2x95x90CW5xe2x80x94Oxe2x80x94, with W5 being H, CH3, or Cl,
m4 and m5 are each independently 0 or 1 in such a manner that oxygen atoms are not linked directly to one another,
MG is selected from: 
X3 is xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond, and
X5 is xe2x80x94COxe2x80x94 or xe2x80x94CH2xe2x80x94.
Preferred compounds of formula I are those for which A3 and A4 have one of the meanings of formula IV, A1 and A2 have one of the meanings of formula VA and A5 to A8 have one of the meanings of formula VB, in which:
MG is cyclohexylene, phenylene, biphenylene, naphthylene or phenanthrylene,
X3 denotes xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94Cxe2x89xa1Cxe2x80x94, or a single bond, in particular xe2x80x94Oxe2x80x94 or a single bond,
Sp2 is straight-chain of formula xe2x80x94(CH2)vxe2x80x94, with v being an integer between 0 and 20, especially preferred being ethylene, propylene, butylene, pentylene, hexylene, heptylene, octylene, nonylene, decylene, undecylene, or dodecylene,
P2 is H, CH2xe2x95x90CW5xe2x80x94 or CH2xe2x95x90CW5xe2x80x94COOxe2x80x94,
P3 is H, CH2xe2x95x90CW5xe2x80x94, CH2xe2x95x90CW5xe2x80x94COOxe2x80x94, W5CHxe2x95x90CHxe2x80x94Oxe2x80x94 or CH2xe2x95x90CW5xe2x80x94Oxe2x80x94, with W5 being H, CH3, or Cl, and
m4 and m5 are each independently 0 or 1 in such a manner that oxygen atoms are not linked directly to one another.
Other aspects of the present invention are:
a) a liquid crystalline material, especially in the form of a liquid crystalline mixture, (co)polymer, elastomer, polymer gel or polymer network, comprising at least two components, at least one of which is an optically active compound, characterised in that the optically active compound is a butane-1,2,3,4-tetraol derivative of formula I;
b) a liquid crystalline material, especially in the form of a cholesteric mixture, or cholesteric polymer network, comprising at least two components, at least one of which is an optically active compound, characterised in that the optically active compound is a butane-1,2,3,4-tetraol derivative of formula I;
c) a cholesteric polymer network obtainable by copolymerisation of an optically active polymerisable mesogenic mixture comprising:
i) at least one optically-active or/and non-optically-active nematic polymerisable compound which can be chosen from the already reported broad range of optically-active and non-optically-active nematic materials, for example in Adv. Mater. 5, 107 (1993), Mol. Cryst. Liq. Cryst. 307, 111 (1997), J. Mat. Chem. 5, 2047 (1995) or in patent applications U.S. Pat. Nos. 5,593,617 and 5,567,349; GB-A-2 297 556; GB-A-2 299 333; DE-A-19 504 224; EP-A-0 606 940; EP-A-0 643 121 and EP-A-0 606 939; optionally selected from EP-A-0 606 940, EP-A-0 643 121 and EP-A-0 606 939,
ii) at least one optically active dopant of formula I,
iii) an initiator,
iv) optionally a non-mesogenic compound having at least one polymerisable functional group, more optionally a diacrylate compound, and
v) optionally a stabiliser;
d) optically-active polymerisable cholesteric mixtures, essentially consisting of:
i) 70 to 99%, preferably 85 to 95% by weight of at least one non-optically-active polymerisable liquid crystal,
ii) 0.1 to 30%, preferably 1 to 15% by weight of an optically active compound of formula I,
iii) 0.1 to 5%, preferably 0.2 to 2% by weight of a photoinitiator,
iv) 0 to 5%, preferably 0.1 to 1% of a stabiliser, and
e) a cholesteric film obtainable by the steps comprising ordering the above mixture in the monomeric state and in situ UV polymerisation of the resulting ordered mixture.
The inventive optically active compounds disclosed in the foregoing and the following can be prepared by methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der Organischen Chemie, Thieme-Verlag, Stuttgart. In the present case of compounds I the commercially available (R,R)- and (S,S)-dimethyl tartrate are used as starting materials, for example according to the following reaction schemes: 
Different methods can be used for the formation of the sought cholesteric network, starting from the polymerisable coloured cholesteric mixture manufactured as described above. Preferably, transparent substrates, optionally coated with ITO (indium tin oxide), and more preferably glass or plastic substrates, were used. Said substrates carried a layer of rubbed polyimide or polyamide or a layer of coated photopolymer. Said layers are used to orient the molecular helix which forms spontaneously in the cholesteric mixture. To preclude the formation of disclinations, the polymerisable cholesteric mixture was:
coated into a thin film, or
provided between two of the said substrates which were sheared over a small distance until a planar order was obtained, or
capillary filled between two of the said substrates,
then subsequently cured, for example, by UV light, preferably in the presence of a photoinitiator, for example an IRGACURE(trademark). Owing to the strength of the three-dimensional polymer network thus formed, the film may be peeled off and used, for example, as a self-supporting cholesteric polariser.
The reflected colour from the formed cholesteric layer is dependent on the pitch length of the cholesteric helix, said pitch length being itself dependent on the concentration of the optically-active dopant in, for example, a nematic host. For small and high concentrations of the optically-active dopant, the cholesteric network reflects red and blue colours respectively.
The novel optically active butane-1,2,3,4-tetraol derivatives of formula I are highly suitable for producing cholesteric films which can be used in different optical and electro-optical applications.